An MRI apparatus in which an examinee is disposed in a uniform magnetic field space and executing a medical diagnosis on the basis of a nuclear magnetic resonance (hereinafter referred to as NMR) signal is used in a medical institution as in the case of examinations based on conventional CT apparatus and RI apparatus. Particularly, the examination using the MRI apparatus enables various examinations for imaging morphological information and functional information such as metabolism, etc. and has excellent lesion detecting capability.
The MRI apparatus contains a magnetostatic field generating device for forming a uniform magnetic field space, a high-frequency coil and a gradient magnetic field coil. The high frequency coil irradiates an examinee with a high-frequency magnetic field pulse to excite the examinee, and receives an NMR signal emitted from the examinee. The gradient magnetic field coil applies a gradient magnetic field pulse in a predetermined direction to the examinee so that the NMR signal is provided with position information.
The high frequency coil constitutes an LC circuit which is designed to electrically resonate at a resonance frequency co. The gradient magnetic field coil is constructed by winding a conductor in orthogonal three axial directions at high density. When the gradient magnetic field coil is disposed between the high frequency coil and the examinee, it disturbs transmission and reception of high-frequency magnetic field pulses. Therefore, the high frequency coil is normally disposed to be proximate to the examinee, and the gradient magnetic field coil is disposed at the outside of the high frequency coil. Even in such an arrangement, when the high frequency coil and the gradient magnetic field coil are close to each other, local inductive inductance and floating capacity occurs in the high-frequency magnetic field coil in the neighborhood of a position where the coil pattern of the high frequency coil is close to the coil pattern of the gradient magnetic field coil. Accordingly, the high frequency coil is not a uniform LC circuit, and thus there occurs a problem that the high frequency magnetic field generated by the high frequency coil is non-uniform.
In order to solve this problem, the construction that a magnetic shield (high frequency shield) of copper foil or the like is disposed between the high frequency coil and the gradient magnetic field coil is known (patent document 1). However, there occurs a problem that high frequency magnetic field emitted from the high frequency coil to the examinee is weakened by the magnetic coupling between the high frequency shield and the high frequency coil. In order to solve this problem, the patent document 1 discloses that the high frequency shield is attached to the bobbin of the gradient magnetic field coil to increase the interval between the high frequency shield and the high frequency coil and weak the magnetic coupling therebetween.
Furthermore, it has been known in the MRI apparatus that vibration occurs in the gradient magnetic field coil due to Lorentz force when it is operated, and thus it serves as a noise generating source (patent document 1, patent document 2). This noise problem occurs in both of a tunnel structure and an open structure. In order to reduce this noise problem, the patent document 1 discloses an MRI apparatus having a tunnel structure in which the gradient magnetic field coil is sealed in a low-pressure container. The patent document 2 discloses an MRI apparatus having an open structure in which the gradient magnetic field coil is surrounded by a shielding cover.
Patent document 1: JP-A-2001-1998104
Patent document 2: JP-A-2001-299719